It does not matter if you change the locale for a cell or a sheet, if your decimal separator in your global locale is "," there is no way to get "VALUE()" to create a NUMBER from texts like: 25.23 1,333.33
Maybe the VALUE() function can honour locale-settings for a sheet or cel, or else can get an optional "FORMAT" parameter like the TEXT() function?
As of LibreOffice 4.1 there will be the NUMBERVALUE function to address this problem.
See bug 57180 for feature implementation.